Wagner fighters have been ordered to pledge their allegiance to the Russian state following the apparent death of the mercenary group's leader in an air crash.
Experts have said that warlord Yevgeny Prigozhin is “highly likely” to have died in the crash north-west of Moscow on Wednesday.
President Vladimir Putin signed a decree bringing in the requirement after the Kremlin said Western suggestions that Mr Prigozhin had been killed on its orders were an “absolute lie”.
Experts said Mr Putin's introduction of a mandatory oath for employees of Wagner and other private military contractors was a clear move to bring such groups under tighter state control.
The decree, published on the Kremlin website, obliges anyone carrying out work on behalf of the military or supporting what Moscow calls its “special military operation” in Ukraine to swear a formal oath of allegiance to Russia.
Described in the decree as a step to forge the spiritual and moral foundations of the defence of Russia, the wording of the oath includes a line promising to strictly follow the orders of commanders and senior leaders.
The Kremlin has declined to definitively confirm Mr Prigozhin’s death, citing the need to wait for test results.
Russia's aviation authority has said Mr Prigozhin was on board a private jet which crashed with no survivors on Wednesday evening, exactly two months after he led a failed mutiny against army chiefs.
Mr Putin sent his condolences to the families of the dead on Thursday and spoke of Prigozhin in the past tense.
He cited “preliminary information” as indicating that the mercenary group leader and his top Wagner associates had all been killed and, while praising Mr Prigozhin, said he had also made some “serious mistakes.”
Western politicians and commentators have suggested, without presenting evidence, that Mr Putin ordered Mr Prigozhin to be killed to punish him for launching the June 23-24 mutiny against the army's leadership which also represented the biggest challenge to Mr Putin's own rule since he came to power in 1999.
Wagner chief Prigozhin presumed dead in plane crash – in pictures
Kremlin spokesman Dmitry Peskov said on Friday that the accusation and many others like it were false.
“There is now a great deal of speculation surrounding this plane crash and the tragic deaths of the plane's passengers, including Yevgeny Prigozhin. Of course, in the West, all this speculation is presented from a well-known angle,” Mr Peskov told reporters.
“All of this is an absolute lie, and here, when covering this issue, it is necessary to base yourself on facts. There are not many facts yet. They need to be established in the course of investigative actions,” he said.
Russian investigators have opened a probe, but have not yet said what they suspect caused the plane to suddenly fall from the sky.
Nor have they officially confirmed the identities of the 10 bodies recovered from the wreckage.
The Pentagon has said its own initial assessment is that Mr Prigozhin was killed.
Russia's Baza news outlet, which has good sources among law enforcement agencies, has reported that investigators are focusing on a theory that one or two bombs may have been planted on board the plane.
Asked about the future of the Wagner Group, which has a series of lucrative contracts across Africa and a contingent in Belarus training the army there but now appears leaderless, Mr Peskov said: “I can't tell you anything now, I don't know.”
